1. Field of the Invention
The invention relates to a catalytic reactor bed arrangement comprising, in a specified distribution, a plurality of catalysts in one or more fixed bed reactors and a process using the same for oxidation of methanol to formaldehyde. More particularly, the invention relates to (1) a catalytic reaction zone (e.g., one or more catalytic reactor beds) comprising, in a specified distribution, a first catalyst of vanadia-titania and a metal molybdate second catalyst, provided in one or more fixed bed reactors, and (2) a process using the same for oxidizing methanol or methanol containing gas streams (i.e., paper pulp mill waste streams) to formaldehyde (CH2O).
2. Description of the Related Art
The formation of formaldehyde involves the dehydrogenation and oxidation of methanol. One approach for converting methanol to formaldehyde involves oxidizing methanol over a silver catalyst. See, for example, U.S. Pat. Nos. 4,080,383; 3,994,977; 3,987,107; 4,584,412; 4,343,954 and 4,343,954. Typically, methanol oxidation to formaldehyde over a silver catalyst is carried out in an oxygen lean environment. One problem associated with silver catalyzed methanol oxidation is methanol leakage, i.e., high amounts of unconverted methanol.
Accordingly, improved processes for oxidizing methanol to formaldehyde have been developed. These processes use a methanol/air mixture (e.g., a reactant gas feed stream of methanol, excess air and an inert carrier gas) introduced over an iron-molybdate/molybdenum trioxide catalyst. See, for example, U.S. Pat. No. 3,983,073 (conversion of methanol to formaldehyde using Fe2(MoO4)3 and MoO3 having a molar ratio of Mo/Fe from 1.5 to 1.7 and a degree of crystallinity of at least 90%); U.S. Pat. No. 3,978,136 (process for the conversion of methanol to formaldehyde with a MoO3/Fe2O3/TiO2 catalyst wherein the MoO3:Fe2O3 weight ratio is between 1:1 to 10:1 and TiO2 is present between 1 to 90 weight % of total oxides); U.S. Pat. No. 3,975,302 (a supported iron oxide and molybdenum trioxide catalyst wherein the atomic ratio of Mo/Fe is from 1.5 to 5); U.S. Pat. No. 3,846,341 (a shaped and optionally supported iron molybdate type catalyst having high mechanical strength made by reacting ammonium molybdate and ferric molybdate); U.S. Pat. No. 3,716,497 (an optionally shaped iron molybdate type catalyst made by admixing with NH4+A−); U.S. Pat. No. 4,829,042 (high mechanical strength catalyst of Fe2(MoO4)3 and MoO3 together with non-sintered Fe2O3); U.S. Pat. No. 4,024,074 (interaction product of Fe2(MoO4)3, MoO3 and bismuth oxide for catalyzing oxidation of methanol to formaldehyde); U.S. Pat. No. 4,181,629 (supported catalyst of iron oxide and molybdenum oxide on silica, alumina and the like); U.S. Pat. No. 4,421,938 (a supported catalyst of at least two oxides of Mo, Ni, Fe and the like); and U.S. Pat. No. 5,217,936 (a catalyst of a monolithic, inert carrier and oxides of molybdenum, iron and the like).
In comparison to the silver catalyzed processes, iron-molybdate/molybdenum trioxide catalyzed processes produce higher yields of formaldehyde. Iron-molybdate, Fe2(MoO4)3, in combination with molybdenum trioxide, MoO3, constitute the metal oxide phases of exemplary commercially available metal oxide catalysts suitable for oxidizing methanol to formaldehyde. During the oxidation of methanol to formaldehyde, the Fe2(MoO4)3/MoO3 catalyst can be generated in situ from physical mixtures of pure molybdenum trioxide, MoO3, and ferric oxide, Fe2O3. See co-pending application designated by U.S. Provisional Ser. No. 60/081,950 of Wachs, et al. Entitled “In Situ Formation of Metal Molybdate Catalysts,” filed Apr. 15, 19098, incorporated herein by reference in its entirety. The molar ratio MoO3/Fe2O3 of these catalysts may be varied. Typically, such catalysts used in industrial and commercial applications contain an excess of MoO3. Thus, for example, the molar ratio MoO3/Fe2O3 may vary from 1.5/1 to 12/1 or more. Excess MoO3 is provided to ensure that sufficient amounts of Fe2(MoO4)3 are formed in situ (from the mixture of Fe2O3 and MoO3) for efficiently oxidizing methanol to formaldehyde in high yields.
Unfortunately, the use of excess MoO3 in conjunction with Fe2O3 or other metal oxides and/or metal molybdates is problematic. Oxidizing methanol to formaldehyde using a metal molybdate/molybdenum trioxide type catalyst, e.g., Fe2(MoO4)3/MoO3, is a highly exothermic process. The heat released during the oxidation reaction increases the catalyst and/or the fixed bed reactor temperature producing “hot spots” on the catalyst surface. These hot spots reach temperatures high enough to volatilize the MoO3 species present within metal molybdate/molybdenum trioxide type catalysts. Thus, MoO3 is sublimed from the hot spots so formed.
The sublimed MoO3 species migrate downstream (e.g., within an exemplary fixed bed reactor housing the catalyst) towards cooler regions of the fixed bed reactor or the like. Typically, the downstream migration of sublimed MoO3 species is facilitated by the incoming flow of the reactant gas feed stream containing, for example, methanol, air, and an optional inert carrier gas fed into the inlet end of a fixed bed reactor. The migrated MoO3 species crystallize in the cooler downstream regions of the fixed bed reactor, for example, in the form of MoO3 crystalline needles. Over time, the needle formation accumulates and ultimately obstructs the flow of the reactant gas feed stream through the fixed bed reactor. Thus, build up of MoO3 crystals/needles in the downstream region causes a substantial pressure drop in the reactant gas feed stream flow rate as the reactant gas feed stream is directed downstream. This pressure drop impedes the efficient oxidation of methanol to formaldehyde. See, for example, U.S. Pat. Nos. 3,983,073 (col. 1, lines 35-52); and 4,024,074 (col. 1, lines 60-68); and U.K. Patent No. 1,463,174 (page 1, col. 2, lines 49-59) describing the aforementioned volatility problem. See also, “Fluidized bed improves formaldehyde process,” C&EN, pp. 37-38 (Nov. 3, 1980; Popov, et al., “Study of an Iron-Molybdenum Oxide Catalyst for the Oxidation of Methanol to Formaldehyde,” Institute of Catalysis, Siberian Branch of the Academy of Sciences of the USSR, Novosibinsk, Transcript from Kiretika & Kataliz, Vol. 17, No. 2, pp. 371-377, March-April, 1976; E. M. McCarron III, et al.; “Oxy-Methoxy Compounds of Molybdenum (VI) and their Relationship to the Selective Oxidation of Methanol Over Molybdate Catalysts, Polyhedron, Vol. 5, No. ½, pp. 129-139 (1986); and L. Cairati et al., “Oxidation of Methanol in a Fluidized Bed Fe2O3—MoO3 Supported Silica,” Chemistry and Uses of Molybdenum, Proceedings of the Fourth International Conference, CLIMAX MOLYBDENUM COMPANY, H. F. Baum and P. C. H. Mitchell, Editors, pp. 402-405, Aug. 9-13, 1982.
Often, the MoO3 needle formation that occurs in the downstream region of the fixed bed reactor is so excessive that the reactor must be shut down, the needles cleaned out, and fresh catalyst charged therein. These steps unnecessarily increase the time, cost, inefficiency and/or complexity of operating a fixed bed reactor or the like for oxidizing methanol to formaldehyde.
The vanadia-titania (V2O5 supported by TiO2) supported catalyst is a catalyst that can also selectively oxidize methanol to formaldehyde. Unfortunately, this catalyst has a disadvantage associated with its use. The vanadia-titania catalyst exhibits an extremely high catalytic activity. Due to this high catalytic activity, this catalyst continues to oxidize formaldehyde into carbon monoxide especially when a high concentration of formaldehyde is available. Consequently, the yield of formaldehyde is undesirably lowered.
Accordingly, there would be an advantage to provide a catalytic reactor bed arrangement and a process using the same that substantially alleviates, and/or eliminates the aforementioned crystallization problems associated with metal molybdate catalysts containing volatile Mo/MoO3 species while simultaneously alleviating and/or eliminating the aforementioned undesirable continued oxidation of formaldehyde to carbon monoxide associated with vanadia-titania catalysts.
Further, (1) silver catalysts, (2) supported catalysts such as those containing silicon dioxide, non-sintered Fe2O3, bismuth interaction products, silica, and/or alumina, (3) catalysts containing zinc, zinc carbonates and/or indium, (4) shaped catalysts, and (5) the like are often prohibitively expensive to use. Accordingly, there remains a need for a catalytic bed reactor arrangement and a method using the same suitable for cost effectively oxidizing methanol to formaldehyde which minimizes the use of one or more of (1) silicon dioxide, (2) non-sintered Fe2O3, (3) interaction products of Fe2(MoO4)3, and MoO3, and bismuth, (4) silica, (5) alumina, (6) shaped catalysts for increasing mechanical strength, (7) catalysts containing Zn(CO3)2.3Zn(OH)2, In(NO3)3.3H2O or one or more of the compounds listed in U.S. Pat. No. 4,421,938, (8) a fibrous carrier material such as silica, or (9) the like.